Thermosetting resins such as cyanate ester (CE) are widely used in the electronics and aerospace industries due to their outstanding adhesive, thermal, mechanical, and electrical properties. However, a drawback of these resins is their brittleness resulting from their highly cross-linked structure, which often restricts their structural applications. Current state of the art techniques include copolymerizing CE with other thermosetting resins, such as epoxy (EP), thereby resulting in a thermostable, processable and tough cyanate ester/epoxy polymer blend which can be produced at a low cost. Another commonly used technique includes adding inorganic nanofillers to the thermosetting resins, which serves to toughen the polymer without compromising their thermal properties.
Carbon nanotubes (CNTs) are deemed to be an ideal material for reinforcing polymer composites due to their low mass density, large aspect ratio (typically between 300-1000), and superior mechanical properties. The mechanical properties of CNT-reinforced composites are improved because CNTs have strength (10-63 GPa) far superior to most thermosetting matrices, and even carbon fibers (about 250 MPa). Furthermore, the nanoscale size of CNTs enables them to be applied as reinforcements in low-dimensional (e.g., 2-D) structures, e.g., polymer fibers, foams, and films, where other conventional microscale fillers would be too large for inclusion. CNTs can also be used to produce multifunctional structural composites with unique thermal, electrical, and optical properties.
Good mechanical properties of CNT/polymer composites, such as a tensile strength of 500-2000 MPa and modulus of 15-169 GPa, have recently been achieved using special nanotubes and/or unconventional processing techniques. With conventional composite processing techniques and common CNTs, however, the properties of resultant CNT-reinforced composites, in particular, those of common thermosetting matrices, have been far inferior than theoretically predicted. For example, Zhu et al. (Nano Lett. 2003, 3, 1107) reported a 30% increase in Young's modulus (from 2.03 to 2.63 GPa) and a 14% increase in tensile strength (from 83.2 to 95.0 MPa) for epoxy composites reinforced with 1 wt % fluorinated single-walled carbon nanotubes (SWNTs).
The lower than expected improvement in mechanical properties of CNT/polymer composite can be partially attributed to the poor nanotube dispersion and nanotube/matrix stress transfer. Due to strong van der Waals forces between the nanotubes, CNTs are usually bundled which can result in inter-tube slippage with applied stress and poor mechanical properties of CNT composites. Furthermore, the graphene structure of CNTs is atomically smooth and highly hydrophobic so that stress transfer to a typical polymer composite matrix, which is usually relatively polar, is poor.
To exploit the high mechanical properties of nanotubes in composites, the nanotubes have to be well-dispersed and the nanotube/matrix interface has to be strong. However, there remain challenges for an effective method to disperse carbon nanotubes such as single-walled carbon nanotubes (SWNTs) into individuals or small bundles, as well as the achievement of strong nanotube/matrix interfacial strength, both of which are needed to exploit the excellent mechanical properties of CNTs in structural composites. For widespread industrial application, it would be desirable to produce CNT-reinforced composites using conventional composite processing methods which exploit the ease of processability of polymers, as well as readily available CNTs and polymer matrix materials.
Therefore, there is a need for an improved method to disperse carbon nanotubes, in particular, a method to disperse carbon nanotubes in a thermosetting resin.